Phytanic acid is a saturated fatty acid found predominantly in red meat and dairy products and may contribute to increases in prostate cancer risk that are observed with higher intakes of these foods. We constructed a novel summary measure of phytanic acid intake and prospectively examined its association with prostate cancer risk in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study—a cohort of Finnish male smokers aged 50–69 years. Diet was assessed at baseline in 27,111 participants using a validated 276-item dietary questionnaire. Since phytanic acid is not currently included in food composition tables, we used the published phytanic acid content of 151 major food items to estimate total daily intake. During up to 21 years of follow-up, a total of 1,929 incident prostate cancer cases (including 438 advanced cases) were identified. Higher phytanic acid intake, though unrelated to the risk of localized disease [relative risks (RR) and 95% confidence intervals (CI) for increasing quartiles of intake = 1.00 (ref), 0.83 (0.68–1.01), 0.76 (0.62–0.94) and 0.91 (0.74–1.13); p trend = 0.23], was associated with increased risks of advanced prostate cancer [RR and 95% CI = 1.00 (ref), 1.43 (1.09–1.89), 1.31 (0.99–1.75) and 1.38 (1.02–1.89); p trend = 0.06]. This association appeared to be driven predominantly by phytanic acid obtained from dairy products (particularly butter). Our study indicates that phytanic acid may contribute to previously observed associations between high-fat animal foods (particularly dairy products) and prostate cancer risk, although some caution is warranted as it may be acting as a surrogate marker of dairy fat.
Higher intakes of dairy products and red meat have been linked with increased risks of prostate cancer in several studies.1–13 For example, a meta-analysis of 10 prospective cohort studies reported that the highest consumers of dairy products had increased risks of both total and advanced disease,3 and a separate meta-analysis of 11 case–control studies showed a 68% increase in prostate cancer risk among men who consumed the highest quantities of milk.13 It has been suggested that total or saturated fat (for both dairy and red meat), calcium and dairy protein/insulin-like growth factor 1 (for dairy), and iron, heterocyclic amines and nitrates/nitrites (for red meat) might explain these associations; however, these individual factors have not been consistently associated with risk.14–16 Therefore, it remains unclear exactly how these foods influence prostate carcinogenesis.
Phytanic acid is a 3-methyl saturated branched-chain fatty acid found predominantly in dairy products and red meat; no phytanic acid is found in foods exclusively of vegetable origin.17 Ruminant animals, including cows, sheep, and goats, contain bacteria in their intestinal tracts that degrade chlorophyll into phytol, which is then metabolized to phytanic acid. Phytanic acid is subsequently deposited in the adipose tissues and milk of these animals. Some fatty fish also accumulate significant levels of phytanic acid; although it is unclear why this occurs, one possibility is that these fish are high-level predators of plankton eaters. Humans obtain phytanic acid exclusively from the aforementioned dietary sources, as they are unable to release phytol from chlorophyll.18 Interest in phytanic acid grew when significant accumulation was reported in the serum and tissues of patients with Refsum's disease—an autosomal recessive disorder characterized by retinitis pigmentosa, chronic neuropathy and cerebellar ataxia.19 It was also noted that phytanic acid blood levels fell considerably when dairy products and ruminant fats were eliminated from these patients' diets.19
The potential importance of phytanic acid in prostate cancer is underscored by evidence that the branched-chain fatty acid oxidation pathway is selectively upregulated in these tumors. α-Methylacyl-CoA racemase (AMACR)—an enzyme that is critical for proper phytanic acid metabolism—is so consistently overexpressed in prostate cancers relative to benign tissues that it is now used clinically as a marker to identify prostate cancer in ambiguous biopsies.20 Two studies to date have investigated whether higher circulating concentrations of phytanic acid are associated with prostate cancer risk, and both studies found suggestive positive associations.21, 22 Furthermore, reports indicate that blood levels of phytanic acid differ significantly between meat eaters, vegetarians and vegans,23 and are correlated with intake of dairy servings,21 cheese,23 dairy fat,22, 23 beef22 and fat from beef.22
These provocative findings suggest that phytanic acid may contribute to the observed associations between dairy product and red meat intake and prostate cancer risk. We report results from the first investigation of the relationship between phytanic acid intake—estimated using food frequency questionnaires in conjunction with the reported phytanic acid content of 151 food items17—and prostate cancer risk. This analysis is based on a large prospective cohort study of Finnish male smokers that accrued more than 1,900 incident prostate cancer cases during up to 21 years of follow-up.
Material and Methods
The Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study was a randomized, double-blind, placebo-controlled trial that tested whether daily supplementation with β-carotene (20 mg) and/or vitamin E (50 mg DL-α-tocopheryl acetate) reduced the incidence of lung and other cancers. Details about study design, methods, participant characteristics and compliance have been reported,24 as have the main trial findings for selected cancers.25 Briefly, 29,133 participants meeting all eligibility criteria at entry (male resident of southwestern Finland aged 50–69 years who smoked five or more cigarettes per day) were successfully randomized between 1985 and 1988. Reasons for exclusion included a prior history of cancer (other than nonmelanoma skin cancer or carcinoma in situ), serious illness or refusal to discontinue use of vitamin E, vitamin A or β-carotene supplements in excess of predefined amounts. The trial ended on April 30, 1993 after 5–8 years of active intervention (median, 6.1 years), and ascertainment of morbidity and mortality end points continued thereafter. The present analysis is based on 27,111 cohort subjects with complete baseline dietary information. Person-years of observation were calculated from the date of randomization to the date of prostate cancer diagnosis, death or April 30, 2006, whichever came first. The institutional review boards of both the National Public Health Institute of Finland (currently National Institute for Health and Welfare) and the US National Cancer Institute approved the study, and written informed consent was obtained from each participant before randomization.
Incident cases of prostate cancer (ICD-9 code 185) were identified through the Finnish Cancer Registry, which provides close to 100% case ascertainment nationwide.26 For cases diagnosed through April 1999, medical records were reviewed centrally by two independent clinical oncologists for diagnostic confirmation and staging. Information on prostate cancer cases diagnosed after this point in time was derived exclusively from the Finnish Cancer Registry. A total of 1,929 prostate cancer cases with complete dietary information were identified during up to 21 years of follow-up time (median = 17 years); of these, 781 had localized disease, 438 had advanced disease [Stage III (T3a-b, N0, M0) or IV (T4, N0, M0; any T,N1,M0; any T, any N, M1) of the tumor-node-metastasis staging system, as defined by the American Joint Committee on Cancer,27 and/or those with a Gleason grade of 8 or higher], and 710 had unknown stage and Gleason grade.
Before randomization, all subjects were asked to provide detailed demographic, smoking and occupational information, to give a history of medical examinations and physician-confirmed diseases and to complete a 276-item dietary questionnaire that ascertained both frequency of intake and portion size for the previous 12 months. A color picture booklet was provided to each participant in order to assist with portion size estimation. The dietary questionnaire was developed specifically for use in the ATBC trial, and was validated against food consumption records in a pilot study conducted in middle-aged Finnish men.28 An overnight fasting blood sample was collected from virtually all participants at baseline, and serum concentrations of α-tocopherol, β-carotene and retinol were determined using high-performance liquid chromatography.29
Estimation of phytanic acid intake
Information on phytanic acid is not available in the Finnish nutrient database. We therefore estimated its consumption using previously published information on the phytanic acid content of 151 major food items (including milk and milk products, fats and oils, meat and meat products and fish).17 A list of phytanic acid-rich foods that were commonly consumed in the ATBC cohort, as well as their assigned phytanic acid content, is given in Table 1. We approximated the amount of phytanic acid present in cream-based products, sour milk products, several types of sausages and herring since these values were not available in the literature. The major contributors to the cream-based products category were cultured half cream/sour cream and cream-based ice cream (both 12% fat); since the phytanic acid content of ruminant products is proportional to the total fat content of that food item, we assigned this category a phytanic acid value that was 25% of that reported for double cream (whipping cream, 48% fat).17 Low-fat milk was assigned half the phytanic acid value of whole milk, and sour milk products were given a value equivalent to low-fat (2.5% fat) yoghurt since the major contributor to this category was kefir (also 2.5% fat). Individual types of sausages were assigned phytanic acid values that were proportional to their beef content; sausages with unknown beef content were assigned a conservative phytanic acid value that was 15% of the one assigned to beef. Herring was assigned the phytanic acid content of Canadian herring meal, as reported by Ann Moser of the Kennedy Krieger Institute (personal communication). Daily intake of phytanic acid was computed by multiplying the reported consumption of each relevant food item (g/day) by the phytanic acid content of that food item (mg/g of food), and then summing across all commonly consumed foods.
Table 1. Average daily intake (mean and SD), assigned phytanic acid values and average phytanic acid derived from commonly consumed food items in the ATBC Study
Phytanic acid from foods that were infrequently eaten in the cohort (mean intake < 100 g/year), including camembert, goat, blue, cream and cottage cheeses, was not included in any of the models presented in the article. This is because risk estimates obtained from models that included and excluded these contributions were very similar.
Subjects were divided into quartiles of phytanic acid based on the distribution in the entire cohort. Cox proportional hazards models were used to determine relative risks (RR) and 95% confidence intervals (CI) for prostate cancer among each quartile of intake, with the lowest category serving as the referent group. Tests for linear trend were carried out by taking the median value of each quartile and modeling as a continuous variable. A base model was specified a priori and included age, smoking dose and duration, trial intervention group and energy intake, with the latter adjustment based on the residual method.30 Addition of body mass index (BMI), personal history of diabetes or benign prostatic hyperplasia, family history of prostate cancer, physical activity level, alcohol consumption and intake of fruit and vegetables, lycopene, α-tocopherol, vitamin D or calcium (all entered as categorical variables) to this model did not alter risk estimates by more than 10%. However, education level and total fat intake (in combination) met the criterion for confounding specified above, and these variables were included in all final models.
We evaluated effect modification by disease severity (localized vs. advanced), trial intervention assignment (α-tocopherol, no α-tocopherol, β-carotene, no β-carotene), family history of prostate cancer (yes, no) and BMI (<25, 25–29.9, >30 kg/m2), as well as by age, baseline serum levels of α-tocopherol and β-carotene, baseline dietary intake of fruits and vegetables, carotenoids, flavonoids, vitamin E, vitamin C and selenium, smoking dose and duration, alcohol consumption and follow-up time (all split at the median value) using stratified analyses. Interactions were tested for statistical significance by including cross-product terms in the appropriate multivariate models.
There was no departure from the underlying proportional hazards assumption (p-value>0.05). All analyses were conducted with SAS (version 8.2, SAS Institute Inc., Cary, NC), and statistical tests were two-tailed with significance levels set at p < 0.05.
Butter was the predominant source of phytanic acid in this cohort, with hard fatty cheeses and whole milk also making significant contributions (Table 1). In comparison, red meat and fatty fish contributed relatively little to phytanic acid intake. Participants in the highest quartile of phytanic acid intake were older, slightly leaner, reported smoking for a longer period of time, consumed less alcohol, were less well educated and less physically active and more likely to report a family history of prostate cancer than individuals in the lowest quartile of intake (Table 2). Intake of calories, total and saturated fat and calcium increased, whereas consumption of iron, fruits and vegetables, α-tocopherol and lycopene decreased, across successive quartiles of phytanic acid intake. The energy-adjusted correlation coefficients between phytanic acid and total fat, saturated fat and dairy fat were 0.47, 0.87 and 0.95, respectively.
Table 2. Baseline characteristics (means and proportions) according to quartiles of estimated phytanic acid intake—the ATBC Study
In multivariate models, increasing phytanic acid intake was associated with a significantly higher risk of advanced, but not localized, prostate cancer (Table 3). A threshold effect was apparent, with risks of advanced disease elevated 43%, 31% and 38% in the second, third and fourth quartiles, respectively, all compared to the reference group. We performed several sensitivity analyses to determine the robustness of the observed associations. Since there is some evidence that marine fatty acids are protective against prostate cancer,31 we re-calculated the summary phytanic acid index without contributions from fish and evaluated its association with prostate cancer—risk estimates were essentially identical to those reported in Table 3 (for advanced prostate cancer, RRs and 95% CIs for increasing quartiles of phytanic acid intake excluding contributions from fish = 1.0 (referent), 1.42 (1.08–1.87), 1.33 (1.00–1.76) and 1.39 (1.02–1.89), p trend = 0.05). We also performed a lag analysis in which 169 incident prostate cancer cases (of which 88 were advanced) diagnosed during the first 5 years of follow-up were removed; the observed associations were similar to those obtained using the full set of cases (data not shown).
Table 3. Relative risks (RR) and 95% confidence intervals (CI) for prostate cancer risk according to quartiles of estimated phytanic acid intake in the ATBC Study, 1985–2006
Analyses stratified by various indicators of oxidative stress yielded no statistically significant interactions (all interaction p-values > 0.05), although there were suggestions of larger phytanic acid-advanced prostate cancer associations in older and overweight men, in those who reported consuming more alcohol, and among men with the lowest intake of fruits and vegetables (Table 4). The association was also stronger among men with the lowest intakes of total flavonoids (for highest versus lowest phytanic acid quartile, RR = 1.82, 95% CI: 1.13–2.93, p trend = 0.05) or vitamin C (RR = 1.79, 95% CI: 1.13–2.82, p trend = 0.02). Unexpectedly, the relation between phytanic acid and advanced disease appeared most pronounced among men randomized to receive the trial vitamin E or β-carotene supplement, as well as in participants with higher pre-randomization serum levels of β-carotene.
Table 4. Relative risks (RR) and 95% confidence intervals (CI) for advanced prostate cancer according to quartiles of estimated phytanic acid intake, stratified by selected factors—the ATBC Study, 1985–2006
We also examined whether phytanic acid consumption was a stronger predictor of prostate cancer risk than total dairy product intake, individual dairy foods, dairy fat, calcium, a composite measure of red meat, specific red meat items and fish (Table 5). Models containing dairy products were adjusted for dietary calcium (<1,000, 1,000–1,499, 1,500–1,999, >2,000 mg/day) because for some of these foods, its inclusion altered risk estimates by 10% or more. Total dairy intake was not associated with increased risks of total nor advanced prostate cancer. Higher consumption of cheese and butter, however, was each linked with statistically significant elevations in the risk of advanced disease, and in each instance, the magnitude of the risk estimate was similar to that for phytanic acid. Notably, these associations were independent of the >60% elevations in risk observed in men ingesting 2,000 mg or more of dietary calcium per day. With the exception of lowfat milk, which was inversely related to advanced tumors, none of the other specific dairy foods, red meat items nor fish were associated with prostate cancer risk. With respect to dairy fat, higher intake was linked with borderline statistically significant increases in the risk of advanced prostate cancer (for highest versus lowest quartile, RR = 1.31, 95% CI: 0.99–1.73, p trend=0.21), although this association was attenuated with further adjustment for calcium (shown in Table 5).
Table 5. Relative risks (RR) and 95% confidence intervals (CI) for prostate cancer risk according to quartiles of dairy product and red meat intakes in the ATBC Study, 1985–2006
To determine whether a specific group of foods was responsible for the association between phytanic acid intake and advanced prostate cancer, we removed contributions from each major food category listed in Table 1 one at a time from the summary measure and re-evaluated its relation to risk (Fig. 1). Exclusion of phytanic acid consumed in butter eliminated the previously observed positive association, as did removal of phytanic acid from all dairy products combined. Removal of contributions from cheese and beef somewhat attenuated risk estimates, whereas exclusion of phytanic acid from cream, milk, yogurt, sour milk products, sausages or fish had little effect.
In this large prospective study, higher estimated intake of phytanic acid was associated with a significant elevation in the risk of advanced prostate cancer. This association was driven principally by phytanic acid obtained from butter, as removal of contributions from this food item eliminated any relationship between phytanic acid and disease risk.
Two studies have examined whether elevated phytanic acid concentrations measured in blood are associated with prostate cancer risk. In a small study conducted in the United States, serum phytanic acid was significantly elevated in men with prostate cancer (n = 49) as compared to unaffected controls (n = 55) (0.10 ± 0.06 and 0.08 ± 0.03 mg/100 mL, respectively; p-value = 0.04).21 In addition, serum levels were significantly positively correlated with self-reported dairy consumption (r = 0.24, p-value = 0.01) and somewhat correlated with overall meat intake (r = 0.16, p-value = 0.09), although there was no discrimination between red meat and other types of meat products. A large European nested case–control study involving 566 incident prostate cancer cases and 566 matched controls demonstrated no association between plasma phytanic acid and total prostate cancer risk, although stratified analysis revealed significant increases in risk among men who had fasted for at least 3 hr prior to blood draw.22 These elevations were not observed for advanced disease, however, which differs from our results. In the same study, plasma phytanic acid levels most strongly correlated with dairy fat intake, as well as butter, beef, fat from butter and fat from beef.
We evaluated whether individual dairy products and red meat were related to prostate cancer risk. Butter and cheese—the two most concentrated sources of phytanic acid—were each linked with significant elevations in advanced disease, and the magnitude of the associations in each quartile were similar to those for phytanic acid. Daily intake of 2,000 mg or more of calcium was also significantly associated with an increase risk of advanced prostate cancer. Importantly, adjustment for calcium did not attenuate the aforementioned findings pertaining to butter and cheese. Our results differ somewhat from a previous investigation of dairy product intake and prostate cancer in the ATBC Study. Mitrou et al. observed a strong, positive association between calcium intake and total, but not advanced disease and reported no relation between individual nor total dairy product consumption and risk after controlling for calcium.8 The discrepancy in results may be due to the fact that we classified advanced disease based on TNM stage and Gleason grade, whereas Mitrou et al.'s definition was based solely on the former criterion. In our study, there was no association between red meat and prostate cancer risk, which is consistent with a prior ATBC publication based on only 184 cases.32
In humans, the metabolism of phytanic acid occurs predominantly in peroxisomes, where reactive oxygen species are generated during multiple rounds of β-oxidation.33 Excessive levels of phytanic acid could amplify β-oxidation activity and overwhelm the activity of endogenous antioxidant defense enzymes, leading to oxidative stress, subsequent DNA damage and disease. Our findings that the phytanic acid-advanced prostate cancer association was most evident in subgroups of men with low fruit and vegetable intake, higher alcohol consumption or excess body weight—all indicators of increased oxidative burden—seems to support such a mechanism. It is unclear, however, why the aforementioned association was stronger in participants randomized to receive the trial vitamin supplements or in those with higher prediagnostic serum β-carotene levels. Since none of the statistical tests for interaction were significant, our exploratory subgroup findings could be due to chance and should be interpreted with caution.
Phytanic acid may also increase prostate cancer through its activity as a ligand for the peroxisome proliferator-activated receptor (PPAR)-α, which is a regulator of lipid metabolism, cell proliferation, differentiation, adipogenesis, inflammatory signaling and apoptosis.34 Another explanation concerns the highly lipophilic nature of phytanic acid, which makes it readily incorporated into the phospholipid bilayer of biological membranes. Once integrated, its bulky structure may alter the conformational state of membrane proteins, affecting fluidity and possibly signal transduction.35 Finally, an in vitro study showed that treatment with phytanic acid and its α-oxidation product, pristanic acid, markedly increased protein levels of AMACR—an enzyme that plays an essential role in the peroxisomal β-oxidation of phytanic acid—in human androgen-sensitive prostate carcinoma cells.36 While AMACR is currently considered to be a diagnostic marker of prostate cancer, it might play a direct role in the initiation or progression of these tumors.
Strengths of this study include its prospective design, the large number of prostate cancer cases available for analysis, our use of a stringent definition for advanced disease and our ability to examine a wide range of confounders, including other potentially harmful compounds (i.e., calcium, iron) found in ruminant products. Finland has the highest per capita consumption of milk and milk products in the world,37 making the ATBC cohort an ideal population to study with regards to phytanic acid intake and disease risk. In support of this, the average estimated daily phytanic acid intake in our study—130 mg—is higher than the 50–100 mg per day supplied by the typical Western diet.38
One limitation is that we were unable to disentangle the effects of phytanic acid from dairy fat, as they were very highly correlated (r = 0.94). Studies have shown marginally significant increases in the risk of advanced prostate cancer among men with higher intake of dairy fat or high-fat dairy foods,8, 39, 40 although the magnitude and significance of these findings was attenuated in studies that additionally adjusted for calcium or utilized calibrated dietary intake values that accounted for measurement error.8, 39 Another limitation is that phytanic acid is not currently included in Finnish (nor American) food composition databases, and we therefore relied on the measured phytanic acid content of 151 food items as reported by Brown et al.17 Since Brown et al. determined the major sources of phytanic acid in the British diet, there were several commonly consumed foods in our study population (including sour milk products, different types of sausages, and herring) for which no phytanic acid value was available; however, only 3% of phytanic acid was consumed in foods with unreferenced amounts. Exposure misclassification due to our estimation of these values, our reliance on a single measure of dietary intake at baseline (which precluded assessment of secular changes in diet during the long follow-up period) and the inherent limitations of food frequency questionnaires41 likely attenuated any true associations. Finally, circulating concentrations may be a more precise indicator of phytanic acid status, although blood may not be available and expensive assays may not be feasible in many large epidemiologic studies. An additional consideration is that food frequency questionnaires capture habitual dietary patterns whereas serologic biomarkers are influenced by temporal variations in exposure and may only reflect recent intake.42 Nevertheless, previous investigations have reported relatively strong correlations between serum phytanic acid levels and dietary intake of phytanic acid-rich foods,21–23 indicating that dietary measures should estimate biochemical status reasonably well. A nested case–control study of serum phytanic acid levels and prostate cancer risk in the ATBC cohort is currently underway; preliminary analysis of the correlation between our dietary phytanic acid index and serum levels in a random subset of 20 controls shows a correlation coefficient of 0.48 (p-value = 0.03).
In conclusion, this study suggests that phytanic acid consumed in high-fat dairy products is associated with significantly elevated risks of aggressive prostate cancer, although some caution is warranted as these effects may be attributed in part to dairy fat. Continued evaluation of relationships between phytanic acid, dairy fat and prostate cancer risk in racially/ethnically diverse populations and in nonsmokers is warranted, as is the investigation of phytanic acid and dairy fat with respect to other malignancies and chronic diseases that have been linked with high dairy product and red meat consumption.